U.S. patent application number 17/297753 was filed with the patent office on 2022-02-10 for iron-aluminum-based plated steel sheet for hot press forming, having excellent hydrogen delayed fracture properties and spot welding properties, and manufacturing method therefor.
The applicant listed for this patent is POSCO. Invention is credited to Yeol-Rae CHO, Sang-Heon KIM, Seong-Woo KIM, Jin-Keun OH.
Application Number | 20220040957 17/297753 |
Document ID | / |
Family ID | |
Filed Date | 2022-02-10 |
United States Patent
Application |
20220040957 |
Kind Code |
A1 |
OH; Jin-Keun ; et
al. |
February 10, 2022 |
IRON-ALUMINUM-BASED PLATED STEEL SHEET FOR HOT PRESS FORMING,
HAVING EXCELLENT HYDROGEN DELAYED FRACTURE PROPERTIES AND SPOT
WELDING PROPERTIES, AND MANUFACTURING METHOD THEREFOR
Abstract
The present invention provides an iron-aluminum-based plated
steel sheet, and a manufacturing method therefor, the
iron-aluminum-based plated steel sheet comprising a base steel
sheet and a plated layer formed on the surface of the base steel
sheet, wherein the alloy plated layer comprises: a diffusion layer
comprising an Fe--Al-based intermetallic compound having a cubic
structure; and an alloyed layer formed on the diffusion layer and
composed of an alloy phase differing from that of the cubic
structure, the thickness of the diffusion layer is 3-20 .mu.m, and
the thickness of the diffusion layer is greater than 50% of the
total thickness of the plated layer.
Inventors: |
OH; Jin-Keun; (Gwangyang-si,
Jeollanam-do, KR) ; KIM; Seong-Woo; (Gwangyang-si,
Jeollanam-do, KR) ; KIM; Sang-Heon; (Gwangyang-si,
Jeollanam-do, KR) ; CHO; Yeol-Rae; (Pohang-si,
Gyeongsangbuk-do, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
POSCO |
Pohang-si, Gyeongsangbuk-do |
|
KR |
|
|
Appl. No.: |
17/297753 |
Filed: |
November 29, 2019 |
PCT Filed: |
November 29, 2019 |
PCT NO: |
PCT/KR2019/016766 |
371 Date: |
May 27, 2021 |
International
Class: |
B32B 15/01 20060101
B32B015/01; C22C 38/32 20060101 C22C038/32; C22C 38/28 20060101
C22C038/28; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C22C 38/00 20060101
C22C038/00; C23C 2/12 20060101 C23C002/12; C23C 2/28 20060101
C23C002/28; C23C 2/40 20060101 C23C002/40 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 30, 2018 |
KR |
10-2018-0152573 |
Nov 29, 2019 |
KR |
10-2019-0156854 |
Claims
1: An iron-aluminum-based plated steel sheet for hot press forming,
the iron-aluminum-based plated steel sheet comprising: a base steel
sheet; and a plating layer formed on a surface of the base steel
sheet, wherein the plating layer includes: a diffusion layer
including a Fe--Al-based intermetallic compound having a cubic
structure; and an alloying layer formed on the diffusion layer and
having a crystal structure different from the cubic structure,
wherein a thickness of the diffusion layer is 3 .mu.m to 20 .mu.m,
and the thickness of the diffusion layer exceeds 50% of a total
thickness of the plating layer.
2: The iron-aluminum-based plated steel sheet of claim 1, wherein a
thickness of the plating layer is 5 .mu.m to 20 .mu.m.
3: The iron-aluminum-based plated steel sheet of claim 1, wherein
the plating layer includes, by wt %, 0.0001% to 7% of silicon (Si),
1.1% to 15% of magnesium (Mg), a balance of aluminum (Al), and
other inevitable impurities, when a remaining alloy composition
excluding an Fe content diffused from the base steel sheet is
100%.
4: The iron-aluminum-based plated steel sheet of claim 1, wherein
the base steel sheet includes, by wt %, 0.04% to 0.5% of carbon
(C), 0.01% to 2% of silicon (Si), 0.01% to 10% of manganese (Mn),
0.001% to 1.0% of aluminum (Al), 0.05% or less of phosphorus (P),
0.02% or less of silicon (S), 0.02% or less of nitrogen (N), a
balance of iron (Fe), and other inevitable impurities.
5: The iron-aluminum-based plated steel sheet of claim 4, wherein
the base steel sheet further includes, by wt %, one or more of
0.01% to 4.0% of the sum of one or more selected from the group
consisting of chromium (Cr), molybdenum (Mo), and tungsten (W),
0.001% to 0.4% of the sum of one or more selected from the group
consisting of titanium (Ti), niobium (Nb), zirconium (Zr), and
vanadium (V), 0.005% to 2.0% of copper (Cu)+nickel (Ni), 0.001% to
1.0% of antimony (Sb)+tin (Sn), and 0.0001% to 0.01% of boron
(B).
6: A hot press formed member obtained by hot press forming the
iron-aluminum-based plated steel sheet according to claim 1,
wherein the thickness of the diffusion layer is 90% or more of the
total thickness of the plating layer.
7: The hot press formed member of claim 6, wherein a content of
diffusible hydrogen in the hot press formed member is 0.1 ppm or
less, and a spot welding current range of the hot press formed
member is 1 kA or more.
8: A method of manufacturing an iron-aluminum-based plated steel
sheet for hot press forming, the method comprising: preparing a
base steel sheet; obtaining an aluminum plated steel sheet through
dipping the base steel sheet in an aluminum plating bath including,
by wt %, 0.0001% to 7% of silicon (Si), 1.1% to 15% of magnesium
(Mg), a balance of aluminum (Al), and other inevitable impurities
to plate the base steel sheet with a coating amount of 10 to 40
g/m.sup.2 per side; and obtaining an iron-aluminum-based plated
steel sheet through online alloying of performing a heat treatment
by maintaining the aluminum plated steel sheet for 1 to 20 seconds
within a heating temperature range of 670.degree. C. to 900.degree.
C. after the plating.
9: The method of claim 8, further comprising spraying aluminum
powder particles onto a surface of the aluminum plated steel sheet,
after obtaining the aluminum plated steel sheet.
10: The method of claim 9, wherein an average particle diameter of
the aluminum powder particles is 5 .mu.m to 40 .mu.m.
11: The method of claim 8, wherein the base steel sheet includes,
by wt %, 0.04% to 0.5% of carbon (C), 0.01% to 2% of silicon (Si),
0.01% to 10% of manganese (Mn), 0.001% to 1.0% of aluminum (Al),
0.05% or less of phosphorus (P), 0.02% or less of sulfur (S), 0.02%
or less of nitrogen (N), a balance of iron (Fe), and other
inevitable impurities.
12: The method of claim 11, wherein the base steel sheet further
includes, by wt %, one or more of 0.01% to 4.0% of the sum of one
or more selected from the group consisting of chromium (Cr),
molybdenum (Mo), and tungsten (W), 0.001% to 0.4% of the sum of one
or more selected from the group consisting of titanium (Ti),
niobium (Nb), zirconium (Zr), and vanadium (V), 0.005% to 2.0% of
copper (Cu)+nickel (Ni), 0.001% to 1.0% of antimony (Sb)+tin (Sn),
and 0.0001% to 0.01% of boron (B).
Description
TECHNICAL FIELD
[0001] The present disclosure relates to an iron-aluminum-based
plated steel sheet for hot press forming, having excellent hydrogen
delayed fracture properties and spot welding properties, and
manufacturing method therefor.
BACKGROUND ART
[0002] In recent years, due to depletion of petroleum energy
resources and high interest in the environment, regulations on
improving fuel efficiency of automobiles have been strengthened. In
terms of materials, reducing a thickness of a steel sheet used in
automobiles may be a method for improving fuel efficiency of
automobiles; however, reducing the thickness of a steel sheet may
cause problems in automobile safety, and thus, in this case,
improvement of strength of the steel sheet should be
facilitated.
[0003] Thus, there has been continuous demand for high-strength
steel sheets, and various kinds of steel sheets have been
developed. However, since these steel sheets have high strength in
themselves, workability thereof is poor. That is, since a product
of strength and elongation for each grade of steel sheet tends to
always have a constant value, when strength of the steel sheet
increases, elongation, an index of workability, decreases.
[0004] In order to solve this problem, a hot press forming method
has been proposed. The hot press forming method is a method of
forming a low temperature structure, such as martensite, in the
steel sheet by forming at a high temperature suitable for forming
and then quenching the steel sheet at a low temperature structure
to increase the strength of the final product. In this case, the
problem of workability may be minimized when manufacturing a member
having high strength.
[0005] However, in the case of the aforementioned hot press forming
method, since the steel sheet has to be heated to have a high
temperature, a surface of the steel sheet is oxidized, which
additionally requires a process of removing oxides on the surface
of the steel sheet after press forming. In order to solve this
problem, patent document 1 was proposed. In this document, a steel
sheet subjected to aluminum plating is used in a process of hot
press forming or heating and quenching after room temperature
forming (briefly, post-heat treatment). Since an aluminum plating
layer is present on the surface of the steel sheet, the steel sheet
is not oxidized at the time of heating, but an increase in
thickness of the plating layer may deteriorate spot welding
properties of the hot press formed member.
[0006] Meanwhile, when subjected to hot press forming, the steel
sheet may have a strength of 1000 MPa or more, and in some cases,
1400 MPa or more. In recent years, the required level for strength
has been further increased, and a steel sheet may have a strength
of 1800 MPa or more. However, when strength of the steel sheet is
increased, the steel sheet becomes sensitive to hydrogen delayed
fracture, so even when a small amount of hydrogen is included, the
steel sheet may be fractured. In addition, in the case of hot press
forming an aluminum-plated steel sheet, Fe diffuses from a base
steel of the steel sheet to the plating layer on the surface,
resulting in alloying in the plating layer, and hydrogen penetrated
during hot press forming cannot easily escape due to the alloying
layer, so that hydrogen resistance properties of the hot press
formed member become inferior.
[0007] (Patent document 1) U.S. Pat. No. 6,296,805
DISCLOSURE
Technical Problem
[0008] An aspect of the present disclosure may provide an
iron-aluminum-based plated steel sheet for hot press forming having
excellent resistance against hydrogen delayed fracture and spot
welding properties, and a manufacturing method therefor.
[0009] The technical problem of the present disclosure is not
limited to the aforementioned matters. Additional problems of the
present disclosure are described in the overall contents of the
disclosure, and those of ordinary skill in the art to which the
present disclosure pertains will not have any difficulty in
understanding the additional problems of the present disclosure
from the contents described in the disclosure of the present
disclosure.
Technical Solution
[0010] According to an aspect of the present disclosure, an
iron-aluminum-based plated steel sheet used for hot press forming
includes: a base steel sheet; and a plating layer formed on a
surface of the base steel sheet, wherein the plating layer
includes: a diffusion layer including a Fe--Al-based intermetallic
compound having a cubic structure; and an alloying layer formed on
the diffusion layer and having a crystal structure different from
the cubic structure, wherein a thickness of the diffusion layer is
3 .mu.m to 20 .mu.m, and the thickness of the diffusion layer
exceeds 50% of a total thickness of the plating layer.
[0011] A thickness of the plating layer may be 5 .mu.m to 20
.mu.m.
[0012] The plating layer may include, by wt %, 0.0001% to 7% of
silicon (Si) and 1.1% to 15% of magnesium (Mg) when a remaining
alloy composition excluding an Fe content diffused from the base
steel sheet is 100%.
[0013] The base steel sheet may include, by wt %, 0.04% to 0.5% of
carbon (C), 0.01% to 2% of silicon (Si), 0.01% to 10% of manganese
(Mn), 0.001% to 1.0% of aluminum (Al), 0.05% or less of phosphorus
(P), 0.02% or less of silicon (S), 0.02% or less of nitrogen (N), a
balance of iron (Fe), and other inevitable impurities.
[0014] The base steel sheet may further include one or more of
0.01% to 4.0% of the sum of one or more selected from the group
consisting of chromium (Cr), molybdenum (Mo), and tungsten (W),
0.001% to 0.4% of the sum of one or more selected from the group
consisting of titanium (Ti), niobium (Nb), zirconium (Zr), and
vanadium (V); 0.005% to 2.0% of copper (Cu)+nickel (Ni), 0.001% to
1.0% of antimony (Sb)+tin (Sn), and 0.0001% to 0.01% of boron
(B).
[0015] According to another aspect of the present disclosure, a hot
press formed member is obtained by hot press forming the above
iron-aluminum-based plated steel sheet, in which the thickness of
the diffusion layer may be 90% or more of the total thickness of
the plating layer.
[0016] According to another aspect of the present disclosure, a
method of manufacturing an iron-aluminum-based plated steel sheet
includes: preparing a base steel sheet; obtaining an aluminum
plated steel sheet through dipping the base steel sheet in an
aluminum plating bath including, by wt %, 0.0001% to 7% of silicon
(Si), 1.1% to 15% of magnesium (Mg), a balance of aluminum (Al),
and other inevitable impurities to plate the base steel sheet with
a coating amount of 10 to 40 g/m.sup.2 per side; and obtaining an
iron-aluminum-based plated steel sheet through online alloying of
performing a heat treatment by maintaining the aluminum plated
steel sheet for 1 to 20 seconds within a heating temperature range
of 670.degree. C. to 900.degree. C. successively, without cooling
the aluminum plated steel sheet, after the plating.
Advantageous Effects
[0017] As described above, in the present disclosure, since the
stable diffusion layer mainly formed of an Fe--Al-based
intermetallic compound having a cubic structure is formed in excess
of 50% of a total thickness of the plating layer on a surface of
the plated steel sheet before hot press forming, hydrogen delayed
fracture properties and spot welding properties of the hot press
formed member may be remarkably improved.
[0018] In addition, the present disclosure may provide a method for
manufacturing an iron-aluminum-based plated steel sheet capable of
reducing manufacturing costs and improving productivity, while
forming a stable diffusion layer mainly formed of an Fe--Al-based
intermetallic compound having a cubic structure, by appropriately
controlling Si and Mg components of a plating bath and process
conditions of an alloying heat treatment and immediately performing
a heat treatment without performing cooling after hot dip aluminum
plating.
DESCRIPTION OF DRAWINGS
[0019] FIG. 1 schematically illustrates a manufacturing apparatus
implementing a manufacturing method according to an aspect of the
present disclosure.
[0020] FIG. 2 is a photograph of a cross-section of an
iron-aluminum-based plated steel sheet manufactured according to
Inventive Example 1, observed with a scanning electron microscope
(SEM).
[0021] FIG. 3 is a photograph of a cross-section of an
iron-aluminum-based plated steel sheet manufactured by Comparative
Example 8, observed with an optical microscope.
[0022] FIG. 4 is a photograph of a cross-section of a plating layer
after hot press forming an iron-aluminum-based plated steel sheet
manufactured according to Inventive Example 1, observed with an
SEM.
[0023] FIG. 5 is a photograph of a cross-section of a plating layer
after hot press forming an aluminum-based plated steel sheet
prepared according to Comparative Example 8, observed with an
optical microscope.
BEST MODE FOR INVENTION
[0024] Hereinafter, the present disclosure will be described in
detail.
[0025] In the present disclosure, it should be appreciated that,
when representing a content of each element, % refers to percent by
weight (wt %), unless otherwise specified. In addition, the ratio
of crystals or tissues is based on area unless otherwise
indicated.
[0026] The inventors of the present disclosure studied in depth
alloy phases of several layers formed of an Fe--Al-based
intermetallic compound formed on an aluminum-plated steel sheet
during conventional hot press forming, and found that alloy phases
(e.g., FeAl(Si), Fe.sub.3Al, etc.) having a cubic structure in the
Fe--Al-based intermetallic compound are stable, while other alloy
phases (e.g., FeAl.sub.3, Fe.sub.2Al.sub.5, etc.) were brittle.
[0027] After a more in-depth study thereof, the inventors of the
present disclosure found that hydrogen was removed from a member
after hot press forming, and here, an aspect in which hydrogen was
removed significantly varied depending on what kind of plating
phase was formed on a surface of a steel sheet before hot press
forming. In particular, it was found that, in the formed alloy
phase, when an orthorhombic crystal phase such as Fe.sub.2Al.sub.5
was formed in a plating layer, movement of hydrogen was blocked and
hydrogen in the steel sheet could not be discharged to the outside.
Based on these results, the inventors of the present disclosure
completed the present disclosure based upon recognition that, when
a diffusion layer mainly formed of an Fe--Al-based intermetallic
compound having a cubic structure is formed to exceed 50% of a
total thickness of the plating layer, the diffusion layer is formed
to be 90% or more in a member after hot press forming, thereby
securing excellent hydrogen resistance properties.
[0028] Hereinafter, an iron-aluminum-based plated steel sheet
according to an aspect of the present disclosure will be described
in detail.
[0029] [Iron-Aluminum-Based Plated Steel Sheet]
[0030] An iron-aluminum-based plated steel sheet according to an
embodiment of the present disclosure includes: a base steel sheet;
and a plating layer formed on a surface of the base steel sheet,
wherein the plating layer includes: a diffusion layer including a
Fe--Al-based intermetallic compound having a cubic structure; and
an alloying layer formed on the diffusion layer and having a
crystal structure different from the cubic structure, wherein a
thickness of the diffusion layer is 3 .mu.m to 20 .mu.m, and the
thickness of the diffusion layer exceeds 50% of a total thickness
of the plating layer.
[0031] In general, when hot press forming is performed on an
aluminum-plated steel sheet, Fe of a base steel sheet is diffused
into an aluminum plating layer having a high Al content, resulting
in an Fe--Al-based intermetallic compound, which is a variety of
hard alloy phases of several layers. In this case, a layer mainly
formed of the Fe--Al-based intermetallic compound having a cubic
structure with excellent resistance to hydrogen embrittlement is
formed on a side close to the base steel sheet, and is stable, but
an alloy phase having a crystal structure such as orthorhombic
system or the like is formed in a direction toward the surface.
However, when such a crystal phase is formed in the plating layer,
movement of hydrogen is blocked so that hydrogen in the steel sheet
cannot be discharged to the outside, degrading hydrogen resistance
properties.
[0032] In order to solve the problem of the related art, in the
iron-aluminum-based plated steel sheet according to an aspect of
the present disclosure, a diffusion layer formed of an Fe--Al-based
intermetallic compound having a cubic structure is formed to meet a
condition of 3 .mu.m to 20 .mu.m and exceeding 50% of a total
thickness of the plating layer, as shown in FIG. 2.
[0033] First, according to an embodiment of the present disclosure,
the diffusion layer may include an Fe--Al-based intermetallic
compound having a cubic structure. In addition, the diffusion layer
may mainly include the Fe--Al-based intermetallic compound having a
cubic structure.
[0034] Specifically, according to an embodiment of the present
disclosure, the diffusion layer may include 50% or more of an
Fe--Al-based intermetallic compound having a cubic structure,
preferably 80% or more, more preferably 90% or more, and most
preferably 95% or more.
[0035] In addition, according to an embodiment of the present
disclosure, the diffusion layer mainly includes an Fe--Al-based
intermetallic compound having a cubic structure and may also
include inevitable impurities and a small amount of other elements
that may be included in a plating bath.
[0036] For example, when Mg is added, Mg may be partially included
in an alloy phase of the Fe--Al-based intermetallic compound in the
diffusion layer, and the diffusion layer may include other alloy
phases including the Fe--Al--Mg-based alloy phase.
[0037] It may be formed of an Fe--Al-based intermetallic compound
having a cubic structure. In the Fe--Al-based intermetallic
compound, the cubic structure is formed in a region with a
relatively high Fe content, and is formed as Fe of the base steel
sheet is diffused into the aluminum plating layer during an
alloying heat treatment. In addition, an alloy phase of the
Fe--Al-based intermetallic compound having a cubic structure may
include FeAl(Si), Fe.sub.3Al, etc. but is not limited thereto.
[0038] If the thickness of the diffusion layer is less than 3
.mu.m, corrosion resistance is inferior, whereas if the thickness
of the diffusion layer exceeds 20 .mu.m, welding properties is
deteriorated. Therefore, the thickness of the diffusion layer is
preferably limited to a thickness of 3 .mu.m to 20 .mu.m, and more
preferably 3.7 .mu.m to 17.9 .mu.m.
[0039] In addition, the thickness of the diffusion layer may be
more than 50% of a total thickness of the plating layer including
the alloying layer, or may be more than 54%. The thickness of the
diffusion layer may be preferably 70% or more, and more preferably
90% or more. When the thickness of the diffusion layer exceeds 50%
of the total thickness of the plating layer, a plating layer
structure in which the thickness of the Fe--Al-based intermetallic
compound having a cubic structure occupies 90% or more in the
plating layer of the hot press formed member may be easily
obtained. From a viewpoint of hydrogen resistance, a higher
proportion of the Fe--Al-based intermetallic compound having a
cubic structure is more preferable, and thus an upper limit thereof
may not be limited.
[0040] In addition, the thickness of the plating layer may be 4.5
.mu.m to 20 .mu.m. If the thickness of the plating layer is less
than 4.5 .mu.m, corrosion resistance may be inferior, while if the
thickness of the plating layer exceeds 20 .mu.m, it may be
difficult to secure a diffusion layer more than 50% in the plating
layer before hot press forming, and if ever, the thickness of the
plating layer may be too thick after hot press forming, making it
difficult to secure spot welding properties. Therefore, in the
present disclosure, the thickness of the plating layer may be 4.5
.mu.m to 20 .mu.m, more preferably 4.5 .mu.m to 18.9 .mu.m.
[0041] According to an embodiment of the present disclosure, the
plating layer may include, by wt %, 0.0001% to 7% of Si, 1.1% to
15% of Mg, a balance of Al, and other inevitable impurities, when a
remaining alloy composition excluding the Fe content diffused from
the base steel sheet is 100%.
[0042] In more detail, in an embodiment of the present disclosure,
Si may be included in an amount of 0.0001% to 7%. Si serves to make
alloying uniform with Fe in the plating layer, and in order to
obtain such an effect, Si needs to be included in an amount of at
least 0.0001% or more. Meanwhile, since Si also serves to inhibit
diffusion of Fe, and thus, if Si is included in excess of 7%,
diffusion of Fe may be excessively inhibited, and thus a desired
plating structure may not be obtained in the present disclosure.
The Si content may be 0.03% to 7%, preferably 1% to 7%, and more
preferably 4% to 7%.
[0043] Meanwhile, Mg serves to improve corrosion resistance of the
plated steel sheet and has an effect of increasing an alloying
rate. In order to obtain the above effect, Mg needs to be included
in an amount of at least 1.1% or more, but if Mg is included in
excess of 15%, welding properties and paintability may be degraded.
Thus, an Mg content may be 1.2% to 12.5%, more preferably 1.1% to
10%, and most preferably 1.1% to 5%. In addition, Mg in the plating
layer tends to diffuse toward the surface, and thus, the Mg content
measured at a depth of 0.5 .mu.m from the surface of the plating
layer with a glow discharge spectrometer (GDS) may be 1 wt % to 20
wt %.
[0044] According to an embodiment of the present disclosure, oxygen
measured at a depth of 0.1 .mu.m from the surface of the plating
layer with a GDS may be 10 wt % or less, and the GDS may be GDS
850A (device name) of LECO of the United States. If oxygen on the
surface of the plating layer exceeds 10 wt %, stains may occur on
the surface of the plated steel sheet, resulting in poor surface
quality. Meanwhile, the less oxygen on the surface of the plating
layer is more advantageous, and thus, a lower limit of the oxygen
content may not be limited.
[0045] According to an embodiment of the present disclosure, the
base steel sheet (base iron), as a steel sheet for hot press
forming, may not be particularly limited when used in hot press
forming. However, as a non-limiting example, the base steel sheet
may have a composition including, by wt %, 0.04% to 0.5% of carbon
(C), 0.01% to 2% of silicon, 0.01% to 10% of manganese (Mn), 0.001%
to 1.0% of aluminum (Al), 0.005% or less of phosphorus (P), 0.002%
or less of silicon (Si), and 0.02% of less of nitrogen (N).
[0046] C: 0.04% to 0.5%
[0047] Carbon (C) may be added in an appropriate amount as an
essential element to increase strength of a heat treatment member.
That is, in order to ensure sufficient strength of the heat
treatment member, C may be added in an amount of 0.04% or more.
Preferably, a lower limit of the C content may be 0.1% or more.
However, if the C content is too high, when a cold rolled material
is produced, strength of a hot rolled material is too high when
cold rolling the hot rolled material, and thus, cold rolling
properties may be significantly deteriorated and spot welding
properties may be significantly lowered. Therefore, in order to
ensure sufficient cold rolling property and spot welding
properties, carbon (C) may be added in an amount of 0.5% or less.
Also, the C content may be 0.45% or less, and more preferably, the
C content may be limited to be 0.4% or less.
[0048] Si: 0.01% to 2%
[0049] Silicon (Si) should be added as a deoxidizer in steel making
and also serves to inhibit an occurrence of a carbide, which has
the greatest effect on strength of the hot rolled press formed
member. In the present disclosure, Si may be added in an amount of
0.01% or more to secure residual austenite by concentrating carbon
at martensite lath grain boundaries after the formation of
martensite in hot press forming. In addition, an upper limit of the
Si content may be set to 2% to ensure sufficient plating properties
when performing aluminum plating on the steel sheet after rolling.
Preferably, the Si content may be limited to 1.5% or less.
[0050] Mn: 0.01% to 10%
[0051] Manganese (Mn) may be added in an amount of 0.01% or more to
lower a critical cooling rate for securing martensite in the hot
press formed member, as well as securing a solid solution
strengthening effect. In addition, the Mn content may be limited to
10% or less in that hot press forming process workability is
secured, manufacturing costs is reduced, and spot welding
properties is improved by appropriately maintaining strength of the
steel sheet. Preferably, the Mn content may be 9% or less, and in
some cases, 8% or less.
[0052] Al: 0.001% to 1.0%
[0053] Aluminum (Al) may increase cleanliness of the steel by
deoxidizing the steel together with Si and may be added in an
amount of 0.001% or more to obtain the above effect. In addition,
the content of Al may be limited to 1.0% or less to prevent the Ac3
temperature from becoming too high, so that heating required during
hot press forming may be performed within an appropriate
temperature range.
[0054] P: 0.05% or Less
[0055] Phosphorus (P) is present as an impurity in the steel and a
less content thereof is advantageous. Accordingly, in the present
disclosure, the P content may be limited to 0.05% or less, and
preferably, may be limited to 0.03% or less. Since a smaller amount
of P is advantageous, there is no need to specifically set an upper
limit of the content. However, excessive lowering the P content may
lead to an increase in manufacturing costs, and in consideration
thereof, a lower limit of the P content may be set to 0.001%.
[0056] S: 0.02% or Less
[0057] Since sulfur (S) is an impurity in the steel and is an
element that inhibits ductility, impacts characteristics and
welding properties of the member, thus, a maximum content of S is
limited to 0.02%, and preferably, to 0.01% or less. In addition, if
a minimum content thereof is less than 0.0001%, manufacturing costs
may increase, so a lower limit of the S content may be set to
0.0001%.
[0058] N: 0.02% or Less
[0059] Nitrogen (N) is an element included as an impurity in the
steel. In order to reduce sensitivity to crack occurrence and
secure impact characteristics during continuous slab casting, and,
a lower content thereof is more advantageous, and therefore, N may
be included in an amount of 0.02% or less. Although it is not
necessary to set a lower limit, the N content may be set to 0.001%
or more in consideration of an increase in manufacturing costs.
[0060] In the present disclosure, optionally as necessary, in
addition to the aforementioned steel composition, 0.01% to 4.0% of
the sum of one or more selected from the group consisting of Cr,
Mo, and W, 0.001% to 0.4% of the sum of one or more selected from
the group consisting of Ti, Nb, Zr, and V, 0.005% to 2.0% of Cu+Ni,
0.001% to 1.0% of Sb+Sn, and 0.0001% to 0.01% of B may be
additionally added.
[0061] The sum of one or more selected from the group consisting of
Cr, Mo and W: 0.01% to 4.0%
[0062] Since the Cr, Mo and W may secure strength and grain
refinement through the improvement of hardenability and
precipitation strengthening effect, one or more thereof may be
added by 0.01% or more based on the total content. In addition, in
order to secure welding properties of the member, the content may
be limited to 4.0% or less. In addition, if the content of these
elements exceeds 4.0%, the effect is saturated, so the content may
be limited to 4.0% or less.
[0063] The sum of one or more selected from the group consisting of
Ti, Nb, Zr, and V: 0.001% to 0.4%
[0064] The Ti, Nb, and V are effective in improving the strength of
the heat treatment member by forming fine precipitates, stabilizing
residual austenite and improving impact toughness by grain
refinement, so one or more thereof may be added by 0.001% or more
based on the total content. However, if the added amount exceeds
0.4%, the effect may be saturated and cost may increase due to
excessive addition of ferroalloy.
[0065] Cu+Ni: 0.005% to 2.0%
[0066] Copper (Cu) and nickel (Ni) are elements that improve
strength by forming fine precipitates. In order to obtain the
aforementioned effect, the sum of one or more of these components
may be 0.005% or more. However, if the value exceeds 2.0%, costs
may be excessively increased, and thus, an upper limit thereof may
be set at 2.0%.
[0067] Sb+Sn: 0.001% to 1.0%
[0068] Antimony (Sb) and tin (Sn) are concentrated on the surface
during an annealing heat treatment for Al--Si plating to inhibit
the formation of Si or Mn oxide on the surface, thereby improving
plating properties. 0.001% or more of Sb+Sn may be added to obtain
such an effect. However, an addition amount of Sb+Sn exceeding 1.0%
may incur excessive ferroalloy cost and cause Sb and Sn to be
dissolved along grain boundaries of a slab to cause coil edge
cracks during a hot rolling process. Thus, an upper limit thereof
is set to 1.0%.
[0069] B: 0.0001% to 0.01%
[0070] The addition of even a small amount of boron (B) improves
hardenability. B segregates along prior-austenite grain boundaries
to inhibit embrittlement of a hot press formed member based on
grain boundary segregation of P and S. Thus, B may be added by
0.0001% or more. If the boron content exceeds 0.01%, the effect is
saturated and causes brittleness at hot rolling, and thus, an upper
limit of the boron content may be set to 0.01%, and in an
implementation example, the boron content may be set to 0.005% or
less.
[0071] The balance other than the aforementioned components may
include iron (Fe) and inevitable impurities, and addition of a
component that may be included in the steel sheet for hot press
forming may not be particularly limited.
[0072] When the iron-aluminum-based plated steel sheet including
the plating layer having the aforementioned layer structure is
heat-treated in a temperature range of 880.degree. C. to
950.degree. C. for 3 to 10 minutes and then hot press formed to
manufacture a hot press formed member, at least 90% of the plating
layer of the hot press formed member may be formed of an
Fe--Al-based intermetallic compound having a cubic structure, and
thus, hydrogen that has penetrated into the steel material may
easily escape during hot press forming and a diffusive hydrogen
content in the steel material may be 0.1 ppm or less, thereby
improving hydrogen resistance properties. In addition, a spot
welding current range satisfies 1 kA or more, so that spot welding
properties may be improved.
[0073] Hereinafter, a method of manufacturing an
iron-aluminum-based plated steel sheet for hot press forming
according to another aspect of the present disclosure will be
described in detail. However, the following method of manufacturing
an iron-aluminum-based plated steel sheet for hot press forming is
only an example and it does not mean that the iron-aluminum-based
plated steel sheet for hot press forming according to the present
disclosure must be manufactured by this manufacturing method. It
should be appreciated that any method may be used to implement each
embodiment of the present disclosure if it satisfies the claims of
the disclosure.
[0074] [Method of Manufacturing Iron-Aluminum-Based Plated Steel
Sheet]
[0075] An iron-aluminum-based plated steel sheet according to
another aspect of the present disclosure may be obtained by
performing aluminum plating on a surface of a hot rolled or cold
rolled base steel sheet with a coating amount of 10.about.40
g/m.sup.2 per side and performing an online alloying treatment by
performing a heat-treatment immediately after the plating
process.
[0076] Step of Obtaining an Aluminum Plated Steel Sheet
[0077] In an embodiment of the present disclosure, a base steel
sheet is prepared and immersed in an aluminum plating bath
including, by wt %, 0.0001% to 7% of Si, 1.1% to 15% of Mg, a
balance of Al, and other inevitable impurities to plate a surface
of the base steel sheet with aluminum with a coating amount of 10
to 40 g/m.sup.2 per side to obtain an aluminum plated steel sheet.
Meanwhile, the coating amount may be more preferably 11 to 38
g/m.sup.2 per side. In addition, annealing may be selectively
performed on the steel sheet before plating.
[0078] Step of Spraying Aluminum Powder
[0079] After the aluminum plating, aluminum powder may be sprayed
on the surface of the aluminum-plated steel sheet as needed. The
aluminum powder not only cools the surface locally but also may
refine surface spangle. Here, when only the surface is locally
cooled by aluminum powder, diffusion of Mg in the plating layer to
the surface in the subsequent online alloying process may be more
inhibited, thereby reducing a Mg oxide generated by diffusion of Mg
to the surface after hot press forming and improving spot welding
properties. In addition, the surface may be uniformly formed after
hot press forming by refining the surface spangle.
[0080] An average particle diameter of the aluminum powder may be 5
.mu.m to 40 .mu.m, and more preferably 10 .mu.m to 30 .mu.m. If the
average particle diameter of the aluminum powder is less than 5
.mu.m, the surface cooling and spangle refinement effect may be
insufficient, whereas if the average particle diameter exceeds 40
.mu.m, the aluminum powder may not be sufficiently dissolved in the
plating layer and remain on the surface, resulting in a surface
quality problem.
[0081] In the present disclosure, a spraying amount of the aluminum
powder may be determined within a limit that satisfies a condition
that a surface temperature does not fall below 640.degree. C. after
powder spraying. If the surface temperature of the steel sheet
after powder spraying falls below 640.degree. C., more power has to
be applied for alloying in a subsequent online alloying heat
treatment, causing an equipment load. The spraying amount of the
aluminum powder is related to the surface temperature of the steel
sheet, but the surface temperature of the steel sheet may vary
significantly depending on process conditions, equipment, and
environmental conditions at the time of implementation, and thus
cannot be uniformly determined. Therefore, since the spraying
amount of aluminum powder satisfying the above condition may be
sufficient, and a specific range of the spraying amount may not be
particularly limited. However, as a non-limiting example, the
aluminum powder may be sprayed within a range of 0.01 to 10 g per 1
m.sup.2 of the aluminum plated steel sheet.
[0082] Step of Obtaining Iron-Aluminum-Based Plated Steel Sheet by
Performing Alloying Heat Treatment
[0083] After the aluminum plating, an online alloying treatment of
performing a heat treatment immediately after performing minimal
air cooling may be performed. In addition, in the case of
selectively spraying aluminum powder after aluminum plating, the
online alloying treatment may be performed immediately after powder
spraying. Here, a heating temperature range during the alloying
heat treatment may be 670.degree. C. to 900.degree. C., and a
holding time may be 1 to 20 seconds.
[0084] In the present disclosure, the online alloying treatment
refers to a process of heat treatment by heating after minimum air
cooling after hot-dip aluminum plating or hot-dip aluminum plating
and aluminum powder spraying, as shown in FIG. 1. In the online
alloying method according to the present disclosure, since the heat
treatment starts before the plating layer is cooled and hardened
after hot-dip aluminum plating, the heat treatment may be performed
within a short time without requiring a separate heating process.
In the general aluminum-plated steel sheet having a thick plating
layer, alloying could not be completed within a short time due to a
thickness thereof, and thus, it was difficult to apply the online
alloying method of performing a heat-treatment immediately after
plating. In contrast, in the present disclosure, alloying of the
aluminum plating layer may be effectively completed despite a short
heat treatment time of 1 to 20 seconds by adjusting the plating
bath components described above and controlling a coating amount of
the aluminum plating layer to 10 to 40 g/m.sup.2 per side.
[0085] The heating temperature is based on a surface temperature of
the steel sheet to be heat-treated. If the heating temperature is
lower than 670.degree. C., insufficient alloying may occur.
Meanwhile, if the heating temperature exceeds 900.degree. C., it is
difficult to cool after alloying, and if the cooling rate is
increased, strength of the base steel sheet may become too high.
Therefore, the heating temperature during the alloying heat
treatment is preferably limited to 670.degree. C. to 900.degree.
C., more preferably 680.degree. C. to 880.degree. C., and most
preferably 700.degree. C. to 800.degree. C.
[0086] Meanwhile, during the alloying heat treatment, the holding
time may be limited to 1 to 20 seconds. In the present disclosure,
the holding time refers to a time during which the heating
temperature (including deviation .+-.10.degree. C.) is maintained
in the steel sheet. If the holding time is less than 1 second, the
heating time is too short to achieve sufficient alloying.
Meanwhile, if the holding time exceeds 20 seconds, productivity may
be too low. Therefore, the holding time during the alloying heat
treatment is preferably limited to 1 to 20 seconds, more preferably
1 to 12 seconds, and most preferably 1 to 10 seconds.
[0087] The formation of the diffusion layer through the alloying
heat treatment depends on a heat treatment temperature and a
holding time and is also affected by the content of Si and Mg
included in the aluminum plating layer. As the amount of Si
included in the aluminum plating layer decreases and the amount of
Mg increases, an alloying rate may increase, and thus the thickness
of the diffusion layer may increase. In the case of performing the
online heat treatment as in the present disclosure, since the heat
treatment time is relatively very short compared to a phase
annealing method, a diffusion layer having a sufficient thickness
cannot be obtained unless the process conditions are precisely
controlled. Accordingly, the inventors of the present disclosure
controlled the Si and Mg contents and heat treatment conditions,
thereby effectively obtaining a diffusion layer having a sufficient
thickness despite a short heat treatment time of 1 to 20
seconds.
[0088] Meanwhile, according to another embodiment of the present
disclosure, a hot press formed member obtained by hot press forming
the iron-aluminum-based plated steel sheet of the present
disclosure may be provided. Here, hot press forming may use a
method generally used in the art. For example, the
iron-aluminum-based plated steel sheet according to the present
disclosure may be heated in a temperature range of 880.degree. C.
to 950.degree. C. for 3 to 10 minutes, and the heated steel sheet
may be hot press formed to have a desired shape using pressing, but
the present disclosure is not limited thereto. In addition, in the
hot press formed member of the present disclosure, a thickness of a
diffusion layer formed of an Fe--Al-based intermetallic compound
having a cubic structure on a surface of a base steel sheet may be
90% or more of the total thickness of a plating layer. In addition,
a composition of the base steel sheet of the hot press formed
member may be the same as a composition of the base steel sheet of
the iron-aluminum-based plated steel sheet described above.
MODE FOR INVENTION
[0089] Hereinafter, the present disclosure will be described more
specifically by way of example. It should be noted that the
following examples are intended to illustrate the present
disclosure in more detail and to not limit the scope of the present
disclosure. The scope of the present disclosure may be determined
by the matters described in the claims and the matters reasonably
deduced therefrom.
Example
[0090] First, a cold-rolled steel sheet for hot press forming
having the composition of Table 1 below was prepared as a base
steel sheet, and aluminum plating and alloying heat treatment were
performed on a surface of the base steel sheet with a plating bath
composition, a plating bath temperature of 660.degree. C., and an
alloying heat treatment conditions shown in Table 2.
[0091] After cooling followed by the alloying heat treatment, a
structure of an alloyed plating layer of the iron-aluminum plated
steel sheet obtained by the above method was observed with an
optical microscope and a scanning electron microscope (SEM) to
identify a thickness of the plating layer and the diffusion
layer.
[0092] In addition, an energy dispersive spectroscopy (EDS)
analysis was performed on a diffusion layer portion of FIG. 2
observed with an alloyed layer portion by the SEM to confirm phases
of Fe.sub.3Al and FeAl having a cubic structure.
[0093] In addition, in FIG. 2, an EDS analysis was performed on a
portion of the alloyed layer formed on the diffusion layer to
detect, by wt %, 48% of Al, 50% of Fe, and 2% of Si, and it was
confirmed that the phase was Fe.sub.2Al.sub.5 having an
orthorhombic structure, not a cubic structure.
TABLE-US-00001 TABLE 1 Element C Si Mn Al P S N Cr Ti B Content (%)
0.22 0.20 1.2 0.03 0.01 0.002 0.0054 0.2 0.03 0.0025
TABLE-US-00002 TABLE 2 Average Plating layer of steel sheet
Aluminum plating condition particle Alloying heat Thickness
Thickness Ratio Coating Si Mg diameter treatment condition of
plating of diffusion of thickness amount content content of Al
Temperature Time layer layer of diffusion Classification
(g/m.sup.2) (Wt. %) (Wt. %) (.mu.m) (.degree. C.) (sec.) (.mu.m)
(.mu.m) layer (%) Inventive 38 6.8 12.5 25 680 10 15.1 8.2 54
Example1 Inventive 38 6.8 12.5 25 800 10 16.8 10.2 61 Example 2
Inventive 38 6.8 12.5 25 880 10 17.8 12.5 70 Example 3 Comparative
38 6.8 12.5 25 600 10 13.3 6.1 46 Example 1 Comparative 38 6.8 12.5
25 950 10 23.3 21.2 91 Example 2 Inventive 37 1.2 10.7 30 680 1
16.2 10.8 67 Example 4 Inventive 37 1.2 10.7 30 800 1 17.8 13.5 76
Example 5 Inventive 37 1.2 10.7 30 880 1 18.9 17.9 95 Example 6
Comparative 37 1.2 10.7 30 680 0.1 15.4 7.2 47 Example 3
Comparative 37 1.2 10.7 30 600 5 15.1 2.9 19 Example 4 Comparative
37 1.2 10.7 30 880 25 22.7 19.4 85 Example 5 Comparative 37 1.2
10.7 30 950 5 27.4 26.8 98 Example 6 Inventive 15 4.2 5.3 10 680 3
5.5 3.8 69 Example 7 Inventive 15 4.2 5.3 10 800 3 5.7 4.7 82
Example 8 Inventive 15 4.2 5.3 10 880 3 5.8 5.6 97 Example 9
Inventive 11 0.03 1.2 16 680 12 4.5 3.7 82 Example 10 Inventive 11
0.03 1.2 16 800 12 5.2 4.5 87 Example 11 Inventive 11 0.03 1.2 16
880 12 5.7 5.4 95 Example 12 Comparative 35 5.5 0 3 800 10 5.7 2.8
49 Example 7 Comparative 50 6.5 7.8 18 800 10 26.7 5.5 21 Example 8
Comparative 35 13.4 1.5 55 800 10 17.8 2.4 13 Example 9 Comparative
35 0 17.8 22 800 10 34.7 18.9 54 Example 10
[0094] Thereafter, each iron-aluminum-based steel sheet was heated
at 930.degree. C. for 6 minutes in an atmospheric atmosphere and
was subsequently subjected to hot press forming to obtain a hot
press formed member. Thereafter, a structure of a plating layer of
the member was observed to measure a content of diffusible hydrogen
and spot welding properties, which are shown in Table 3 below. To
measure the content of diffusible hydrogen, a hydrogen content
discharged by heating the sample to 300.degree. C. was measured
using a gas chromatography technique, and spot welding properties
were evaluated based on ISO 18278 to analyze a current range.
TABLE-US-00003 TABLE 3 Iron-aluminum-based plated steel sheet Hot
press formed member Ratio of Content Ratio of Content Spot
thickness of thickness of welding of diffusible of diffusible
current diffusion hydrogen diffusion hydrogen range Classification
layer (%) (ppm) layer (%) (ppm) (kA) Inventive 54 0.01 99 0.05 1.6
Example1 Inventive 61 0.02 100 0.04 1.6 Example 2 Inventive 70 0.01
100 0.02 1.4 Example 3 Comparative 46 0.02 82 0.24 1.8 Example 1
Comparative 91 0.02 100 0.03 0.6 Example 2 Inventive 67 0.01 96
0.08 1.6 Example 4 Inventive 76 0.007 96 0.07 1.6 Example 5
Inventive 95 0.01 97 0.06 1.4 Example 6 Comparative 47 0.01 75 0.28
1.8 Example 3 Comparative 19 0.01 52 0.52 1.8 Example 4 Comparative
85 0.02 100 0.05 0.4 Example 5 Comparative 98 0.01 100 0.05 0.2
Example 6 Inventive 69 0.02 97 0.06 2.2 Example 7 Inventive 82 0.01
100 0.02 2.0 Example 8 Inventive 97 0.02 100 0.03 2.0 Example 9
Inventive 82 0.01 100 0.03 1.6 Example 10 Inventive 87 0.008 100
0.02 1.6 Example 11 Inventive 95 0.01 100 0.02 1.6 Example 12
Comparative 49 0.01 48 0.58 2.0 Example 7 Comparative 21 0.004 64
0.34 1.6 Example 8 Comparative 13 0.02 35 0.6 0.4 Example 9
Comparative 54 0.01 94 0.08 0.8 Example 10
[0095] As can be seen in Tables 1 to 3, Inventive Examples 1 to 12
satisfy all of the plating bath components and the alloying heat
treatment conditions presented in the present disclosure, and the
ratio of the thickness of the diffusion layer including an alloy
phase of the Fe--Al-based intermetallic compound having a cubic
structure in the plated steel sheet was 50% or more.
[0096] In addition, it can be seen that the diffusive hydrogen
content in the steel was 0.1 ppm or less, and the spot welding
current range satisfied 1 kA or more when the hot press formed
member was manufactured, so that the hydrogen delayed fracture
properties and spot welding properties are excellent.
[0097] However, in Comparative Examples 1 and 4, the alloying heat
treatment temperature was lower than 670.degree. C., and
Comparative Example 1 had a diffusion layer thickness ratio of 50%
or less because the diffusion layer was not sufficiently formed,
and Comparative Example 4 had a diffusion layer having a thickness
less than 3 .mu.m. Accordingly, in the hot press formed members
manufactured with the plated steel sheets of Comparative Examples 1
and 4, the ratio of the thickness of the diffusion layer was less
than 90%, and hydrogen did not easily escape, so that the content
of diffusible hydrogen was 0.1 ppm or more, degrading hydrogen
resistance.
[0098] In Comparative Examples 2 and 6, the alloying heat treatment
temperature exceeded 900.degree. C., and the thickness of the
plating layer and the diffusion layer exceeded 20 .mu.m.
Accordingly, in the hot press formed member, the spot welding point
current range was less than 1 kA, resulting in poor spot welding
properties.
[0099] Meanwhile, Comparative Examples 3 and 5 are cases in which a
holding time during the alloying heat treatment is outside of the
range of the present disclosure. In the case of Comparative Example
3, the heat treatment time was too short to sufficiently form the
diffusion layer, so the ratio of the thickness of the diffusion
layer of the hot press formed member was low as 75%, degrading
hydrogen resistance. In addition, in the case of Comparative
Example 6, the heat treatment time was as long as 25 seconds, so
that the thickness of the plating layer exceeded 20 .mu.m,
resulting in poor spot welding properties.
[0100] Comparative Examples 7, 9 and 10 are examples in which the
Si or Mg content of components of the aluminum plating bath does
not satisfy the conditions of the present disclosure. Comparative
Example 7 is a case in which Mg was not added and Comparative
Example 9 is a case in which Si was added in excess of 7%, and the
diffusion layer was not sufficiently formed due to a low alloying
rate. As a result, the content of diffusible hydrogen in the steel
increased in the hot press formed member, hydrogen resistance
decreased. In addition, in Comparative Example 10, Mg was added in
excess of 15%, so that the plating layer was formed to a thickness
exceeding 20 .mu.m, and accordingly, spot welding properties were
poor.
[0101] Comparative Example 8 is a case in which the amount of
aluminum plating is outside of the scope of the present disclosure,
the thickness of the plating layer increased to 26.7 .mu.m, the
ratio of the thickness of the diffusion layer was reduced to
degrade hydrogen resistance.
[0102] While embodiments of the present disclosure have been shown
and described, it will be apparent to those skilled in the art that
modifications and variations could be made without departing from
the scope of the present disclosure. Therefore, the scope of the
present disclosure is not limited to the embodiments but should be
defined by the appended claims and equivalents thereof.
DETAILED DESCRIPTION OF MAIN ELEMENTS
[0103] 1: HEAT TREATMENT FURNACE [0104] 2: ALUMINUM PLATING BATH
[0105] 3: ALUMINUM POWDER SPRAY DEVICE [0106] 4: ALLOYING HEAT
TREATMENT DEVICE
* * * * *